This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The expected results have potential to make fundamental scientific contributions, while also making direct connections to design and operating strategies for water quality professionals. The research uses basic scientific concepts from quantum chemistry and thermodynamics, and the experimental tasks utilize modern analytical techniques and computational software. The four key research hypotheses are: 1) The biological oxidation of PhACs and their metabolites will occur at the location where the frontier electron density (FED) is highest. FED may be established as an organizing principle for predicting biologically-mediated PhAC transformation. These results may impact the understanding of PhAC mass transport in bioreactors, surface waters, and sediments. 2) PhAC sorption is entropy-driven, endothermic, and diffusion-controlled. Establishing the thermodynamic baseline for PhAC sorption to activated sludge biomass will help to understand available data in a meaningful way. 3) Sorption hysteresis is more pronounced as the particle size distribution shifts toward larger sizes. The results could produce a fundamental understanding for how sorption hysteresis is affected by activated sludge particle size. 4) PhACs or their metabolites located within the interior of larger floc particles are more persistent. When both biodegradation and sorption occur simultaneously, PhACs and their metabolites may become trapped within the interior of activated sludge floc particles, where biological activity is lower.